Mobile terminal apparatus and system selecting method

ABSTRACT

In a first standby state, the reception quality of a signal from a system which has established synchronization is detected. If the detected reception quality deteriorates to be less than a threshold value, the destination of synchronization establishment is changed from a first system to a second system. In a second standby state after this system change, the reception quality of a system having a higher priority order of selection than the second system which has established synchronization is detected. If a third system whose detected reception quality exceeds the threshold value is found, the destination of synchronization establishment is changed to this third system again. After that, the first standby state is returned.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Divisional of application Ser. No. 10/241,566, now U.S. Pat.No. 7,003,315 filed Sep. 12, 2002, which is incorporated in its entiretyherein by reference. This application is also based upon and claims thebenefit of priority from the prior Japanese Patent Application No.2001-310313, filed Oct. 5, 2001, the entire contents of all of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mobile terminal apparatus used in anarea in which services are provided by a plurality of cellular mobilecommunication systems different in communication standards, e.g., adigital cell phone system, analog cell phone system, and PCS (PersonalCommunication System), and to a system selecting method used to selectan optimum system while this mobile terminal apparatus is in a standbystate.

2. Description of the Related Art

Generally, when a mobile terminal apparatus used in a cellular mobilecommunication system is powered on, the apparatus searches for a controlsignal transmitted by a base station of a system desired by the user, onthe basis of a list registered beforehand. The mobile terminal apparatusthen performs processing for establishing synchronization with the basestation transmitting the control signal acquired by the search. Whenthis synchronization is established, the mobile terminal apparatusproceeds to a standby state. Once the apparatus proceeds to this standbystate, no effective messages from the base station can be received overa predetermined period after that. The mobile terminal apparatus holdsthis synchronization established state with respect to the desiredsystem until a “system lost” condition occurs or speech or datacommunication once started is terminated.

With this configuration, however, if the reception quality deterioratesin the standby state, this deteriorated reception quality statecontinues until the “system lost” condition occurs or communication oncestarted is terminated. In this case, if, among peripheral base stationswhich belong to the same system as the base station which hasestablished the synchronization, there is a peripheral base stationhaving higher reception quality than that of the former base station,idle handoff is performed for the latter base station. This can improvethe reception quality. However, if there is no peripheral base stationwhich belongs to the same system as the base station which hasestablished the synchronization, or if no high-quality signals can bereceived from a peripheral base station which belongs to the samesystem, no idle handoff can be performed. Accordingly, the mobilecommunication terminal apparatus in a situation like this keeps standingby with the deteriorated reception quality. This sometimes interfereswith connection control for an outgoing call or incoming call.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in consideration of the abovesituation, and has as its object to provide a mobile terminal apparatuscapable of rapidly eliminating deterioration of the reception qualityeven if idle handoff cannot be performed between base stations in thesame system while the apparatus is in a standby state, and a systemselecting method of the apparatus.

To achieve the above object, the present invention provides a mobileterminal apparatus capable of connecting to a plurality of mobilecommunication systems having different communication standards,comprising a system acquiring section configured to acquire a firstsystem from the plurality of mobile communication systems, and proceedto a first standby state, a first determination section configured to,in the first standby state, detect the reception quality of a signaltransmitted from the first system, and check whether the detectedreception quality satisfies a first condition, and a first systemreacquiring section configured to, if the first determination sectiondetermines that the detected reception quality does not satisfy thefirst condition, acquire from the plurality of mobile communicationsystems a second system whose reception quality satisfies a secondcondition, and proceed to a second standby state. The apparatus furthercomprises a second determination section configured to, in the secondstandby state, detect the reception quality of a signal transmitted froma system except for the second system, and check whether there is asystem whose detected reception quality satisfies a third condition, anda second system reacquiring section configured to, if the seconddetermination section determines that there is a system whose detectedreception quality satisfies the third condition, acquire the systemmeeting the third condition and return to the first standby state.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention and, together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a schematic view showing the state of a service area in whicha mobile communication terminal apparatus according to a firstembodiment of the present invention is used;

FIG. 2 is a block diagram showing the functional configuration of themobile communication terminal apparatus according to the firstembodiment of the present invention;

FIG. 3 is a flowchart showing the procedure and contents of systemselection control performed by the mobile communication terminalapparatus shown in FIG. 2;

FIG. 4 is a flowchart showing the procedure and contents of first systemreselection control;

FIG. 5 is a flowchart showing the procedure and contents of secondsystem reselection control;

FIG. 6 is a flowchart showing the procedure and contents of systemselection control performed by a mobile communication terminal apparatusaccording to a second embodiment of the present invention; and

FIG. 7 is a flowchart showing the procedure and contents of systemselection control performed by a mobile communication terminal apparatusaccording to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(First Embodiment)

FIG. 1 is a schematic view showing the state of a service area in whicha mobile communication terminal apparatus according to a firstembodiment of the present invention is used. In this service area, aplurality of systems (three systems are shown in FIG. 3) operatedindependently of each other form radio areas ES1, ES2, and ES3. Basestations BSS1, BSS2, and BSS3 are installed in radio areas ES1, ES2, andES3, respectively, and are connected to mobile communication switchingapparatuses (not shown) operated by the individual systems.

Examples of the plurality of systems are a digital portablecommunication system using CDMA (Code Division Multiple Access) scheme,a personal mobile communication system called PCS (PersonalCommunication System) using the same CDMA scheme, and an analog cellphone system using an analog communication system represented by AMPS(Advanced Mobile Phone System).

For the sake of convenience of explanation, each system has one radioarea, i.e., the radio area ES1, ES2, or ES3 shown in FIG. 1. Inpractice, however, a plurality of radio areas are formed in each system,and a base station is installed in each of these radio areas.

FIG. 2 is a block diagram showing the functional configuration of themobile communication terminal apparatus according to the firstembodiment of the present invention. This mobile communication terminalapparatus is a multi-mode terminal capable of communication byselectively using a digital mode which uses CDMA and an analog modewhich uses AMPS.

A radio signal transmitted from the base station BSS1, BSS2, or BSS3 isreceived by an antenna 1 and input to a receiving circuit (RX) 3 via anantenna duplexer (DUP) 2. The receiving circuit 3 mixes the radio signalwith a receive local oscillator signal output from a frequencysynthesizer (SYN) 4, thereby down-converting the signal to anintermediate-frequency signal or baseband signal. Note that thefrequency of the receive local oscillator signal generated by thefrequency synthesizer 4 is designated by an output control signal SYCfrom a controller 12.

The received intermediate-frequency or baseband signal is input to aCDMA signal processing unit 6 when digital mode is set as acommunication mode, or to an analog audio circuit 19 when analog mode isset.

For the input received intermediate-frequency or baseband signal, theCDMA signal processing unit 6 performs orthogonal demodulation anddespreading using a spreading code (PN code) assigned to the receivingchannel, thereby obtaining demodulated data. This demodulated data isinput to a speech codec 7. A signal indicating the reception data rateof the demodulated data is input to the controller 12.

The speech codec 7 first expands the demodulated data output from theCDMA signal processing unit 6. This expansion process is performed inaccordance with the reception data rate notified by the controller 12.The speech codec 7 then performs a decoding process using Viterbidecoding and an error correction decoding process. By these decodingprocesses, received digital data of the baseband is reproduced.

A PCM codec 8 performs signal processing in accordance with the type(speech communication or data communication) of communication mode. Thiscommunication mode type is designated by the controller 12. In speechcommunication mode, PCM decoding is performed for the received digitaldata output from the speech codec 7, and an analog reception speechsignal is output. This analog reception speech signal is amplified by areceiver amplifier 9 and output as speech from a loudspeaker 10. In datacommunication mode, the received digital data output from the speechcodec 7 is input to the controller 12. The controller 12 stores thisinput received digital data in a memory 13 and at the same time displaysthe data on a display 15. If necessary, the controller 12 outputs thereceived digital data from an external interface to a portableinformation terminal (PDA: Personal Digital Assistant) or a personalcomputer such as a notebook personal computer (neither is shown).

The analog audio circuit 19 performs analog demodulation for the inputreceived intermediate-frequency signal by using an FM detector or thelike, thereby reproducing an analog reception signal. This analogreception signal is amplified by the receiver amplifier 9 and output asspeech from the loudspeaker 10.

An analog transmission speech signal output from a microphone 11 inspeech communication mode is amplified to an appropriate level by atransmitter amplifier 18. This amplified analog transmission speechsignal is input to the PCM codec 8 when digital mode is selected as thisspeech communication mode, or to the analog audio circuit 19 when analogmode is selected.

The PCM codec 8 performs PCM encoding for the input analog transmissionspeech signal. The generated transmission speech data is input to thespeech codec 7.

This speech codec 7 changes its processing in accordance with whetherspeech communication mode or data communication mode is set. In speechcommunication mode, the amount of energy of the input speech is detectedfrom the input transmission speech data. On the basis of the detectionresult, the transmission data rate is determined. On the basis of thisdetermined transmission data rate, the input transmission speech data iscompressed. The compressed transmission speech data is subjected toerror correction encoding and input to the CDMA signal processing unit6. In digital communication mode, output transmission data from thecontroller 12 is compressed in accordance with the preset transmissiondata rate. The compressed data is subjected to error correction encodingand input to the CDMA signal processing unit 6.

The CDMA signal processing unit 6 first performs spread spectrumprocessing for the input transmission data. This spread spectrumprocessing is done by multiplying the transmission data by a PN codeassigned to the transmitting channel. Digital modulation is thenperformed for the spread transmission data. QPSK is an example of thisdigital modulation. The transmission signal generated by the digitalmodulation is input to a transmitting circuit (TX) 5.

The analog audio circuit 19 performs analog modulation of a carriersignal by using the input analog transmission speech signal. FMmodulation is an example of this analog modulation system. The modulatedcarrier signal is input to the transmitting circuit (TX) 5.

The transmitting circuit 5 synthesizes the input orthogonal modulatedsignal or carrier signal with a transmit local oscillator signalgenerated by the frequency synthesizer 4, thereby converting the signalinto a radio-frequency signal. On the basis of the transmission datarate notified by the controller 12, the transmitting circuit 5 performshigh-frequency amplification only for an effective portion of theradio-frequency signal, and outputs as a transmission radio-frequencysignal. The transmission radio-frequency signal output from thistransmitting circuit 5 is supplied to the antenna 1 via the antennaduplexer 2. The antenna 1 transmits the signal toward a base station(not shown) by burst transmission.

An input unit 14 has dial keys, a transmission key, a power key, an endkey, volume control keys, a mode designating key, and the like. Thedisplay 15 has an LCD and LEDs. The LCD displays, e.g., storedinformation of a telephone directory, a log of incoming and outgoingcalls, and the telephone number and operating state of a terminalapparatus of the communication partner. The LEDs are used to inform theuser of an incoming call and warn the user when a battery 16 requirescharging.

A power circuit 17 generates a predetermined operating power supplyvoltage Vcc on the basis of the output from the battery 16. This powercircuit 17 also has a charging circuit which charges the battery 16.

The controller 12 includes, e.g., a microcomputer as a main controlunit. The controller 12 also includes a communication control sectionwhich makes speech communication or data communication possible byexecuting radio connection control in accordance with an outgoing callor incoming call, a position registration control section, and an idlehandoff control section. In addition, as control sections according tothe present invention, the controller 12 includes a system selectioncontrol section (not shown), a first system reselection control section12 a, and a second system reselection control section 12 b. Thesefunctions are implemented by making the microcomputer execute programs.

When the terminal apparatus is powered on, the system selection controlsection selects a system having a high priority order of selection inaccordance with a list registered beforehand. The system selectioncontrol section acquires a base station which belongs to the selectedsystem, establishes synchronization with this acquired base station, andproceeds to a standby state. This standby state will be referred to as afirst standby state hereinafter.

In this first standby state, the first system reselection controlsection 12 a detects the received field strength (RSSI) and Ec/Io of areception signal coming from the base station of the system which hasestablished synchronization. The first system reselection controlsection 12 a compares the detected RSSI and Ec/Io with threshold valuesAr and Ae, respectively, thereby monitoring the reception quality of thereception signal. This reception quality monitoring is performed foreach slot of the reception signal. If the reception quality continuouslydeteriorates over a predetermined number of slots (e.g., four slots) ormore, the first system reselection control section 12 a executes a firstsystem reselecting process. This first system reselecting processselects a system regardless of the priority order of selection of eachsystem. That is, if a system having high reception quality exists amongsystems having low priority orders of selection, the first systemreselection control section 12 a selects this system, establishessynchronization, and proceeds to a standby state. This standby statewill be referred to as a second standby state hereinafter.

In this second standby state, the second system reselection controlsection 12 b detects the received field strength (RSSI) and Ec/Io of asignal coming from the base station of each system except for the systemwhich has established synchronization. This detection is periodicallyperformed at a predetermined interval (e.g., 3 minutes). The secondsystem reselection control section 12 b compares the detected RSSI andEc/Io with threshold values Br and Be, respectively, thereby determiningthe presence/absence of a system having high reception quality. If asystem having a higher priority order of selection and higher receptionquality than the system which has established synchronization is found,the second system reselection control section 12 b establishessynchronization with this system and proceeds to a standby state. Thisstandby state is the first standby state described above.

The operation of system selection performed by the mobile communicationterminal apparatus configured as above will be explained below.

Assume, for example, that the user turns on the power supply when themobile communication terminal apparatus MS is in the position shown inFIG. 1. The mobile communication terminal apparatus MS initializes theindividual circuits and, under the control of the controller 12, startssystem selection control as follows. FIG. 3 is a flowchart showing theprocedure and contents of the control.

That is, in step 3 a, the controller 12 selects a system having thehighest priority order of selection on the basis of a system listregistered beforehand. In step 3 b, the controller 12 performs controlfor establishing synchronization with a base station which belongs tothis selected system. For example, assume that the system having thehighest priority order of selection is the CDMA portable communicationsystem S1. In this case, the controller 12 selects this CDMA portablecommunication system S1. The controller 12 then acquires the nearestbase station BSS1 from base stations which belong to the selected systemS1. Subsequently, the controller 12 executes control for establishingsynchronization with this acquired base station. When synchronization isestablished, the controller 12 proceeds to the first standby state shownin step 3 c.

After proceeding to the first standby state, in step 3 d the controller12 executes reception quality determination for the first systemreselecting process. FIG. 4 is a flowchart showing the procedure andcontents of the determination.

That is, for each slot of a signal coming from the base station BSS1which has established synchronization, the controller 12 detects thereceived field strength (RSSI) and Ec/Io of the signal in steps 4 a and4 b, respectively. Ec/Io is the ratio of the energy (Ec) of the signalcoming from the base station BSS1 as an object of measurement to thetotal energy (Io) of received signals. The controller 12 compares thedetected RSSI and Ec/Io with the threshold values Ar and Ae in steps 4 cand 4 d, respectively, thereby determining whether the reception qualityhas deteriorated.

Assume that the determination result indicates that both the RSSI andEc/Io are smaller than the threshold values Ar and Ae, respectively.Accordingly, in step 4 e the controller 12 counts up a value a of acounter for counting the number of slots by which deterioration of thereception quality is found. If the value a of the counter after thiscount up is equal to or larger than a preset value, e.g., “4”, thecontroller 12 determines that the reception quality of the signal fromthe base station BSS1 which has established synchronization hasdeteriorated, and proceeds to the first system reselecting process shownin step 3 e.

On the other hand, assume that at least one of the RSSI and Ec/Io islarger than the corresponding one of the threshold values Ar and Ae orthat the value a of the counter is smaller than “4”. In this case, thecontroller 12 determines that the reception quality of the signal fromthe base station BBS1 which has established synchronization has notdeteriorated or has deteriorated only temporarily, and returns to thefirst standby state shown in step 3 c.

After proceeding to the first system reselecting process shown in step 3e, the controller 12 searches a plurality of systems registered in thesystem list for a system having a lower priority order of selection thanthe system S1 which has established synchronization but having receptionquality which is equal to or higher than a predetermined level. If asystem meeting the conditions is detected, the controller 12 acquires abase station of the detected system, establishes synchronization withthis acquired base station, and proceeds to the second standby state.

Assume, for example, that a system meeting the conditions is the systemS3 using AMPS. In this case, the controller 12 selects this AMPS systemS3, acquires the base station BSS3 which belongs to the system S3,establishes synchronization with the acquired base station BSS3, andproceeds to the second standby state.

Accordingly, the mobile communication terminal apparatus MS connected tothe CDMA portable communication system S1 is not left to stand by withthe reception quality deteriorated. The standby state is continued withthe AMPS system S3 having higher reception quality than the system S1.

After proceeding to the second standby state, the controller 12 advancesto step 3 g to execute reception quality determination for the secondsystem reselecting process. FIG. 5 is a flowchart showing the procedureand contents of the determination.

In step 5 a, the controller 12 activates a rescan timer for defining thedetermination period of the reception quality. The time of this rescantimer is set to, e.g., 3 minutes. If the rescan timer times out, in step5 b the controller 12 selects a system in accordance with the priorityorder of selection from the system list. In steps 5 c and 5 d, thecontroller 12 detects the RSSI and Ec/Io, respectively, of a signal fromthe base station of the selected system. In steps 5 e and 5 f, thecontroller 12 compares the detected RSSI and Ec/Io with the thresholdvalues Br and Be, respectively.

Assume that at least one of the RSSI and Ec/Io is smaller than thecorresponding one of the threshold values Br and Be. In this case, thecontroller 12 returns from step 5 g to step 5 b to select a systemhaving the second highest priority order of selection and repeat theprocesses of detecting and determining the RSSI and Ec/Io. If a systemin which both the RSSI and Ec/Io exceed the threshold values Br and Beis found, the controller 12 selects this system and returns to step 3 b.In step 3 b, the controller 12 establishes synchronization with a basestation which belongs to the selected system, and proceeds to the firststandby state.

Assume, for example, that both the RSSI and Ec/Io of a signal from thebase station BSS2 of the PCS system S2 exceed the threshold values Brand Be, respectively. In this case, the controller 12 selects this PCSsystem S2, establishes synchronization with the nearest base stationBSS2 of the system S2, and proceeds to the first standby state. If, forexample, the RSSI and Ec/Io of a signal from the base station BSS1 ofthe CDMA portable communication system S1 exceed the threshold values Brand Be, respectively, the controller 12 reselects this CDMA portablecommunication system S1 and establishes synchronization. That is, thedestination of synchronization establishment of the mobile communicationterminal apparatus MS in this case returns to the CDMA portablecommunication system S1 which is selected when the power supply isturned on and has the highest priority order of selection.

Accordingly, if in the second standby state a system whose receptionquality is higher than a reference value is found among systems havinghigher priority orders of selection than the system which hasestablished synchronization, the destination of synchronizationestablishment is again automatically changed to this detected system.This avoids the inconvenience that the standby state continues for longperiods with respect to the AMPS system S3 having a low priority orderof selection.

Assume that when, of all the systems registered in the system list, thereception quality of each system having a higher priority order ofselection than the AMPS system S3 which has established synchronizationis checked, no system in which both the RSSI and Ec/Io exceed thethreshold values Br and Be, respectively, is found. If this is the case,the controller 12 returns from step 5 g to step 3 f to continue thesecond standby state. Whenever the rescan timer times out, thecontroller 12 performs the second system reselection control describedabove.

The threshold values Br and Be used in the second system reselectingprocess are set to be larger than the threshold values Ar and Ae,respectively, used in the first system reselecting process. Therefore,the AMPS system S3 is changed to the CDMA system S1 or S2 again onlywhen the reception quality of each system as the destination of thechange is sufficiently high. This prevents “ping-pong” system selectionin which the destination of synchronization establishment of the mobilecommunication terminal apparatus MS changes frequently between the AMPSsystem S3 and the CDMA system S1 or S2.

In the first embodiment described above, while the first standbyoperation is performed for a system selected from the system list inaccordance with power-on, the first reselection control section 12 achecks the reception quality (RSSI and Ec/Io) of a signal from thesystem S3 which has established synchronization. If it detects that thereception quality has deteriorated below the threshold values aplurality of times, the system reselection control section 12 a executesa system reselecting process which gives importance to the receptionquality rather than to the priority orders of selection defined in thesystem list.

Accordingly, even in a situation in which handoff between base stationsin a system cannot be performed in the first standby state, thedestination of synchronization establishment can be automaticallychanged to the other system S3 having relatively high reception quality.Since this enables a rapid escape from the deteriorated receptionquality state, radio connection control for an outgoing call andincoming call can be reliably performed. That is, the connectioncompletion ratio improves.

In the first embodiment, in the second standby state in which a systemas the destination of synchronization establishment is changed by thefirst system reselection control section 12 a, the second systemreselection control section 12 b monitors the reception quality of asystem having a higher priority order of selection than the AMPS systemS3 which has established synchronization in this second standby state.If a system S2 or S1 whose reception quality exceeds the thresholdvalues is detected, the destination of synchronization establishment ischanged to this detected system S2 or S1 again.

Accordingly, even when the destination of synchronization establishmentin the standby state is temporarily changed to a system such as the AMPSsystem S3 having a low priority order of selection, if a system such asthe CDMA system S1 having a higher priority order of selection than thesystem after the change and having reception quality higher than areference value is detected, the destination of synchronizationestablishment is again automatically changed to this CDMA system S1. Ifthe reception quality of the CDMA system S1 recovers, the destination ofsynchronization establishment is automatically returned to this CDMAsystem S1 having the highest priority order of selection. Hence, themobile communication terminal apparatus MS can stand by in a systemhaving the highest priority order of selection while maintaining thereception quality.

In the first embodiment as described previously, the threshold values Brand Be used in the second system reselection control are set to belarger than the threshold values Ar and Ae, respectively, used in thefirst system reselection control. This can prevent the destination ofsynchronization establishment of the mobile communication terminalapparatus MS in the standby state from ping-ponging, between a systemhaving a high priority order of selection such as a CDMA portablecommunication system or a PCS system and a system having a low priorityorder of selection such as an AMPS system. This increases the stabilityof the standby state of the mobile communication terminal apparatus MS.Also, since the system reselecting process is not frequently performed,it is possible to reduce the power consumption of the mobilecommunication terminal apparatus MS and so extend the battery life ofthe terminal apparatus.

Furthermore, in the first system reselection control, the receptionquality of a signal from the CDMA system S1 which has establishedsynchronization is checked for each slot. System reselection is executedif this reception quality continuously deteriorates below the thresholdvalues over a predetermined number (e.g., four) of slots. Therefore, thesystem reselecting operation is not easily activated by temporarydeterioration of the reception quality caused by, e.g., fading. Thisrealizes a highly stable and highly effective system reselectingoperation.

(Second Embodiment)

As shown in FIG. 6, while first system reselection control and secondsystem reselection control are performed in a first standby state (step3 c) and a second standby state (step 3 f), respectively, an event whichcan be a trigger of a system reselecting process is monitored in step 6a. Examples of an event which can be a trigger are “system lost” in thestandby state and the termination of communication. The any of theseevents is detected, a system reselecting process is executed. “Systemlost” in the standby state is the inability to receive any effectivemessages in succession for 3 seconds or more in a paging channelreceiving period except for a sleep period of an intermittent receivingoperation.

(Third Embodiment)

In the first embodiment, if a system in which both the RSSI and Ec/Ioexceed the threshold values Br and Be is detected even once during thesecond system reselection control, the destination of synchronizationestablishment is again immediately changed to this detected system.However, as in the first system reselection control, when a system inwhich both the RSSI and Ec/Io exceed the threshold values Br and Be aplurality of times in succession is found, the destination ofsynchronization establishment can again be changed to the detectedsystem.

FIG. 7 is a flowchart showing the procedure and contents of secondsystem reselection control performed by a mobile communication terminalapparatus according to this third embodiment. The same reference symbolsas in FIG. 5 denote the same steps in FIG. 7, and a detailed descriptionthereof will be omitted.

Assume that a system in which both the RSSI and Ec/Io exceed thethreshold values Br and Be in steps 5 e and 5 f, respectively, is found.In this case, in step 7 a a controller 12 counts up a value b of acounter for counting the number of times of improvement of the receptionquality. If this value b of the counter after the count up is smallerthan N, the controller 12 determines that the reception quality of thesystem is still unstable, and the flow returns from step 5 g to step 5c. The controller 12 then selects a system having the second highestpriority order of selection, and repeats the processes of detecting andchecking the RSSI and Ec/Io.

If the value b of the counter after the count up exceeds the presetvalue N, the controller 12 determines that a system having stablyimproved reception quality has been found. So, the flow returns to step3 b, and the controller 12 establishes synchronization with this systemand proceeds to the first standby state.

Since, therefore, a system in which the reception quality is temporarilyimproved is not immediately selected, a highly stable system selectingoperation can be realized. Note that the value of N can be any arbitrarynumber selected from one or more values in accordance with the requiredsystem selection stability.

The present invention is not limited to the above embodiments. Forexample, it is possible to modify, without departing from the gist ofthe invention, the types and number of systems to be reselected, themethod of setting the priority order of selection of each system, theprocedures and contents of the first and second system reselectioncontrol operations, the procedures and contents of detection anddetermination of the reception quality, and the types and arrangementsof mobile communication terminal apparatuses.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit and scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A mobile terminal for connecting to a plurality of mobilecommunication systems, and wherein each mobile communication systemcorresponds to different communication standards, the mobile terminalcomprising: a memory configured to store priority informationcorresponding to the plurality of mobile communication systems; a systemacquiring section configured to acquire a first system from theplurality of mobile communication systems with reference to the priorityinformation stored in the memory, and proceed to a standby state; afirst determination section configured to detect a first receptionquality of a signal transmitted from the first system, and check whetherthe detected first reception quality satisfies a first condition; afirst reacquiring section configured to acquire, from the plurality ofmobile communication systems, a second system whose reception qualitysatisfies a second condition if the first determination sectiondetermines that the first reception quality does not satisfy the firstcondition, and proceed to the standby state; a second determinationsection configured to detect reception quality of a signal transmittedfrom at least one system having higher priority information than that ofthe second system, with reference to the priority information in thestandby state, and check whether there is a third system whose receptionquality satisfies a third condition; and a second reacquiring sectionconfigured to acquire the third system satisfying the third condition ifthe second determination section determines that there is the thirdsystem whose reception quality satisfies the third condition, andproceed to the standby state.
 2. The mobile terminal according to claim1, wherein the first system and the third system acquired by the secondreacquiring section are the same system.
 3. A mobile terminal forconnecting to a plurality of mobile communication systems, and whereineach mobile communication system corresponds to different communicationstandards, the mobile terminal comprising: a memory configured to storepriority information corresponding to the plurality of mobilecommunication systems; a system acquiring section configured to select afirst system from the plurality of mobile communication systems basedupon the priority information stored in the memory, acquire a basestation which belongs to the first system, and proceed to a standbystate; a first determination section configured to detect a firstreception quality of a signal transmitted from the acquired base stationin the standby state, and check whether the first reception qualitysatisfies a first condition; a detection section configured to detectreception quality of a signal transmitted from a base station whichbelongs to a system, other than the first system, of the plurality ofmobile communication systems in the standby state; a first reacquiringsection configured to select, from other mobile communication systems, asecond system whose reception quality is higher than the first receptionquality if the first determination section determines that the firstreception quality does not satisfy the first condition, acquire a basestation which belongs to the selected second system, and proceed to thestandby state; a second determination section configured to detect thirdreception quality of a signal from a base station belonging to a systemhaving higher priority information than that of the second system withreference to the priority information and check whether there is a thirdsystem whose reception quality satisfies a second condition; and asecond reacquiring section configured to acquire a base stationbelonging to the third system if the second determination sectiondetects that there is the third system, and proceed to the standbystate.
 4. The mobile terminal according to claim 3, wherein the firstdetermination section detects the first reception quality of the signaltransmitted from the base station of the first system, and checkswhether the detected reception quality falls below a first threshold;and the second determination section detects the third reception qualityof the signal transmitted from the base station which belongs to thesystem having higher priority information than that of the secondsystem, and checks whether the detected third reception quality exceedsa second threshold higher than the first threshold.
 5. The mobileterminal according to claim 3, wherein the first and seconddetermination sections detect, as the reception quality, the receivedfield strength of a received signal and the ratio (Ec/Io) of the energy(Ec) of a signal received from a base station to be detected to thetotal energy (Io) of received signals, respectively, and compare thedetected received field strength and the Ec/Io with different thresholdto check whether both the received field strength and Ec/Io are not lessthan the threshold.
 6. The mobile terminal according to claim 3, whereinthe first determination section selects the second system if the firstdetermination section determines that the detected reception qualityfails to satisfy the first condition the predetermined number of times.7. The mobile terminal according to claim 3, wherein the seconddetermination section detects, a plurality of times, the receptionquality of the signal from the base station belonging to the systemhaving higher priority than that of the second system, and checkswhether the third reception system satisfies the second condition thepredetermined number of times.
 8. A mobile terminal for connecting to aplurality of mobile communication systems, each mobile communicationsystem corresponds to different communication standards, comprising: amemory configured to store priority information corresponding to theplurality of mobile communication systems; a system acquiring sectionconfigured to acquire a first system from the plurality of mobilecommunication systems with reference to the priority information storedin the memory, and proceed to a standby state; a first determinationsection configured to detect a first reception quality of a signaltransmitted from the first system, and check whether the detected firstreception quality satisfies a first condition; a first reacquiringsection configured to acquire, from the plurality of mobilecommunication systems, a second system whose reception quality satisfiesa second condition if the first determination section determines thatthe first reception quality does not satisfy the first condition, andproceed to the standby state; a second determination section configuredto detect reception quality of signals from systems having higherpriority information than that of the second system in order withreference to the priority information in the standby state, and checkwhether there is a third system whose reception quality satisfies athird condition; and a second reacquiring section configured to acquirethe third system satisfying the third condition if the seconddetermination section determines that there is the third system whosereception quality satisfies the third condition, and proceed to thestandby state.
 9. The mobile terminal according to claim 8, wherein thesecond determination section detects reception quality of signals fromthe systems in the order of descending priorities, with reference to thepriority information and checks whether there is the third system whosereception quality satisfies the third condition, and stops the detectionof the reception quality of signals, if there is the third systemsatisfying the third condition.
 10. The mobile terminal according toclaim 8, wherein the second determination section is activated atpredetermined time intervals.